In the rat brain, large-scale cell proliferation, migration, and differentiation proceed during the first few weeks after birth. These cellular processes underlie the dynamic changes in brain morphology, electrophysiology and function that occur during postnatal development. Superimposed upon these age-dependent processes are the more subtle changes in brain structure and function that occur in the young as a result of experience and/or environmental influence. The objective of this proposal is to (a) define, for the first time, the normal pattern of gene expression in a specific brain region during its postnatal development, and (b) to initiate studies designed to evaluate the effects of experience on gene activity in brain. Estimates of the complexity of gene expression will be obtained both at the level of transcription and translation by RNA/DNA hybridization under conditions of RNA excess. Nuclear RNA and polyadenylated mRNA will be obtained from cerebral cortex at several stages during postnatal development and after rearing animals in different environments. The possibility that a complex population of mRNA lacking poly (A) sequences may exist in brain will also be investigated. The complexity and diversity of these RNA populations will be estimated by saturation hybridization using 3H-labelled nonrepetitive DNA as probe. The RNA copy frequency distribution will also be investigated using reassociation kinetic analysis of DNA synthesized from RNA by reverse transcriptase (cDNA). The results of this study should (a) provide much needed insight into the role played by the genome in the establishment and maintenance of brain structure and function and (b) yield an index of brain plasticity in terms of gene expression.